1. Field of the Invention
The present invention relates to an integrated metallurgical reactor for smelting oxides of metal, such as iron oxide, that is a compact reactor housing formed to have separate chambers to separate the key processes necessary for the manufacture of a variety of steels and irons from a variety of iron-bearing feedstocks, and having means for separate slag removal in each of the process regions to permit optimum performance and adjustment of the process in each stage.
2. Description of the Prior Art
In recent years there has been a significant increase in research aimed at developing new direct smelting processes particularly those using coal as the reductant. There are many motives for this effort most of which relate to the two broad categories of economic benefit and environmental concern.
A recent survey entitled "Coal Based Ironmaking" by Smith and Corbett (Ironmaking and Steelmaking, 14 (2), 1987), outlines a range of technologies and illustrates three basic flow sheet types covering single stage, two stage and three stage processes. In the single state configuration, ore, coal and oxygen are introduced into a single processing vessel, and the products are metal, slag and off gases. In the two stage configuration, ore and smelter off gases are introduced into a reduction unit and the reduced ore proceeds to the second stage (melting unit) where melting energy is provided by coal and oxygen. The separate use of smelter off gas reduces the overall energy requirement compared to single stage operation. The three stage configuration provides for separate reduction, gasification and melting, and potentially has the lowest energy requirement by converting excess heat in the melted off gas into chemical energy by the oxidation of carbon by CO.sub.2 and/or H.sub.2 O. Unfortunately, these theoretical benefits are not always realized since several direct smelting processes based on the three stage principle require the use of coke which has serious energy inefficiencies associated with its production. The three stage process is inherently the most efficient and encompasses reduction, gasification and melting. Smith and Corbett illustrate four conceptual flow sheets for the three stage process concept.
An alternative way of considering process efficiency is to examine the use of the carbon in the reductant as it is employed to remove oxygen from the various oxidation levels of iron oxide species. The two major natural iron ore minerals are hematite (Fe.sub.2 O.sub.3) and magnetite (Fe.sub.3 O.sub.4) while the stable intermediate phase, wustite (FeO) does not occur naturally but plays a major role in the reduction sequence from iron ore to metal. The importance of wustite is illustrated by the relative energy requirements in the three step reduction sequence hematite.fwdarw.magnetite wustite.fwdarw.metal. At 800.degree. C., the relative heats of reaction are 1 for hematite to magnetite, 3 for magnetite to wustite (Fe.sub.0.947 O) and 7.5 for wustite to iron.
The reduction can be considered in three steps. The least efficient, from a carbon utilization point of view, is that in which all oxygen is removed by elemental carbon to produce carbon monoxide. The reaction with elemental carbon generally takes place at high temperatures as in the lower regions of the blast furnace. At lower temperatures it is possible to reduce iron oxides with gases rich in carbon monoxide and two cases can be considered. In one case the natural oxides are reduced to wustite by gaseous carbon monoxide generated in a subsequent reduction step using elemental carbon and the final gaseous product is a mixture of carbon monoxide and carbon dioxide.
In the other case the excess carbon monoxide is used to reduce wustite to metallic iron and the final gaseous product is carbon dioxide. These three steps are represented by equations 1, 2 and 3 for hematite and by equations 4, 5 and 6 for magnetite. EQU Fe.sub.2 O.sub.3 +3 C.fwdarw.2 Fe+3 CO (Eq. 1) EQU Fe.sub.2 O.sub.3 +2 C.fwdarw.2 Fe+CO.sub.2 +CO (Eq. 2) EQU 2 Fe.sub.2 O.sub.3 +3 C.fwdarw.4 Fe+3 CO.sub.2 (Eq. 3) EQU Fe.sub.3 O.sub.4 +4 C.fwdarw.3 Fe+4 CO (Eq. 4) EQU Fe.sub.3 O.sub.4 +3 C.fwdarw.3 Fe+CO.sub.2 +2 CO (Eq. 5) EQU Fe.sub.3 O.sub.4 +2 C.fwdarw.3 Fe+2 CO.sub.2 (Eq 6)
The mass ratio of required carbon to iron produced, and the mass ratio of generated carbon monoxide to iron produced, are as follows.
______________________________________ Eqn. No. C/Fe CO/Fe ______________________________________ 1 .320 .750 2 .214 .25 3 .161 0 4 .286 .667 5 .214 .333 6 .143 0 ______________________________________
The minimum theoretical carbon requirement is for reduction involving one mole of magnetite with two moles of carbon. The carbon requirements for each reaction (C.sub.1) relative to this minimum (C.sub.6) are as follows.
______________________________________ Eqn. No. = i C.sub.i /C.sub.6 ______________________________________ 1 2.238 2 1.497 3 1.126 4 2.000 5 1.497 6 1.000 ______________________________________
The carbon requirement for total reduction of magnetite by carbon (Eqn. 4) is 100% greater than that required for the optimal reduction of magnetite (Eqn. 6. Since the carbon requirement is provided by coal it can be seen that the cost for coal to carry out the various reduction sequences represented by equations 1 through 6 above varies over a range of two to one. There is consequently a considerable economic incentive to seek processes that will utilize the reaction sequences that minimize carbon requirements.
Eketorp and Brabie (Scand J. Met 3, 1974) have pointed out that in high temperature reduction smelting systems the reduction of iron oxides by carbon generates only CO and that energy recovery from the product gas is an important component in smelting reactor design considerations. As shown by Smith and Corbett (Ironmaking and Steelmaking, 14 (2), 1987) there are two fundamental approaches to this energy recovery concept; one seeks to maximize heat recovery to the melt by post reduction combustion while the other seeks to effectively use the carbon monoxide component in the off gas mixture for pre-reduction of the iron oxide feed.
Two recent approaches to direct smelting processes seek to maximize the energy recovery by high post reduction combustion and heat transfer. In one approach as disclosed by Warner in U.S. Pat. No. 4,701,217, 1987, two large furnaces of a geometric configuration similar to those employed in nonferrous reverberatory furnace practice are operated side by side. All the combustible gases produced by coal volatilization and CO generation by the high temperature smelting reduction of iron oxide feedstocks flow to the combustion section of one of the furnaces and are combusted with preheated air and/or oxygen introduced through overhead lances in a manner similar to that employed in reverberatory furnace practice.
Warner achieves a high degree of heat transfer by a design which provides for a very large clean metal surface area of the order of 200 m.sup.2 for a plant capable of producing 2000 tons/day of hot metal.
In Warner's approach, the second furnace is employed for carbon absorption into the metal from a lump coal feed covering essentially the total surface area of 400 m.sup.2. Due to the relatively poor mass transfer from solid coal to liquid metal, both a large surface area and high metal velocities are required. The text of the patent recommends that the method be performed with a very large proportion of molten carrier material circulation to molten metal produced with a preferred ratio of 288:1. Thus, for a 2000 ton/day plant a circulation rate of 24,000 ton/hr. is required. In an alternative operating mode for the dual reverberatory furnace system fine coal can be introduced onto the flowing metal surface with or without the simultaneous addition of fine iron ore feedstocks.
In yet another recent approach, Innes et al., in a paper entitled "Direct Smelting of Iron Ore in a Liquid Iron Bath - The HIsmelt Process" presented at the 71st Steelmaking Conference, Toronto, April 1988, describe a process in which coal fines and iron ore fines are injected into a molten iron bath together with top and bottom injection of oxygen and other process gases. The use of an iron bath as a reaction medium is common practice in steelmaking as in the Q-BOP process and bottom injection of coal fines is practiced in both the European and Japanese steel industries. The HIsmelt technology relies entirely on the simultaneous injection of fine iron ore and coal and the resulting simultaneous reaction sequences in the iron bath. These simultaneous reaction sequences involve: coal devolatization; char formation; partial combustion; carbon dissolution into metal; slag formation; sulfur release from the coal; sulfur partitioning between gas, slag and metal phases as well as iron ore melting and reduction. Excellent reaction kinetics are favored by the high temperature of the bath and the high degree of turbulence arising from the injection of the various components.
The top injection of oxygen is a key component of this technology and involves top blown lance technology similar to that employed in BOF practice. In the HIsmelt case, the top surface of the bath is highly agitated and the gases being released are highly combustible so that the top injection of oxygen results in an intense post combustion zone above the bath under conditions which favor good heat transfer to the melt. However, due to the combined injection of iron ore and coal into the bath, the largest possible generation of gases takes place and fully effective combustion at the top surface of the bath may not be achieved. Under these circumstances the overall heat transfer efficiency will decrease. It is clear that the overall effectiveness of the system is strongly dependent on the degree of post combustion achieved and the proportion of combustion energy returned to the iron bath.
In the field of continuous steelmaking, earlier work has been reviewed by Fukuzawa (Trans. Nat. Res. Inst. for Metals, Vol. 27, No. 2, 1985). Motivation for seeking effective continuous steelmaking technology is driven by increasing demands for higher quality steel products and the energy and environmental benefits of reducing BOF slag volumes. Current practice to reduce slag volumes has led to separate pretreatment steps before final BOF refining, and has achieved improved quality at the expense of increasing the number of batch processing steps required. In continuous steelmaking, the extent of conversion is a function of length as opposed to batch operation where it is a function of time. This introduces the concept of carrying out the separate steelmaking refining steps at different positions in a continuous flow channel.
Fukuzawa reviews seven different continuous steelmaking process types, all of which use a linear flow of molten metal and aim to achieve treatment of the metal stream in one pass. The treatment chemistry and methods for introducing regents and/or gas streams follow conventional practice. Furthermore, it is generally recognized that mass transfer is the rate limiting step and consequently conventional engineering principles can be applied for the determination of the flow, mixing and residence time criteria needed to achieve a desired degree of treatment.
Continuous steelmaking has not yet been established at the full commercial scale, and some critical practical aspects have therefore not arisen in the various small scale experimental studies carried out to date. In the refining of steel, high temperatures and aggressive slags are involved, and place a heavy burden on the refractory walls of the containing vessel. Refractory problems in conventional practice, which involves a sequence of batch operations, can be resolved by intermittent refractory patching or partial relining between major relining shutdowns. These options are not available in the case of continuous steelmaking, and the development of long life refractory systems will be an important aspect of continuous steelmaking process technology.
As will be revealed later, these problems can be reduced by the judicious use of existing technology including refractory surfaces that are cooled sufficiently to cause the formation of a frozen layer of metal which separates the molten metal from the refractory wall.